The cathodes of DC high-pressure discharge lamps, such as HBO lamps (mercury vapor lamps) or XBO lamps (xenon lamps), for example, generally consist of tungsten, which has been doped with thorium oxide. The proportion of thorium oxide is in this case approximately 0.4 to approximately 3% by weight. Since thorium oxide is a radioactive substance, radioactivity can also be found in thoriated tungsten electrodes. Legislative regulations control the handling of radioactive substances. If a critical activity is reached, different identification requirements and measures are necessary when handling these substances. Doping cathodes with thorium oxide has the function of lowering the work function at the cathode peak, as a result of which a lower cathode peak temperature can be achieved during lamp operation. Associated with this, the cathode burnback is reduced over the lamp life, which becomes apparent to the user in a positive way in a lower decrease in the utilized flux or the utilized radiated light.
An increase in the lamp power generally requires an enlargement of the cathode dimensions in order to keep the temperature and the electrode burnback associated therewith as low as possible. In the case of discharge lamps with a power of up to approximately 5 kW, the entire cathode or the cathode head can be produced from thoriated material without the limit value of the activity being exceeded. At powers of more than 8 kW, this is no longer possible.
In particular in the case of cathodes which comprise a core and a sheath, these cathodes have not proven to be mechanically stable when they have a conventional configuration. In particular during lamp starting, it may arise that the cathode is torn apart.